Problem: Luis is 3 times as old as Christopher and is also 12 years older than Christopher. How old is Christopher?
We can use the given information to write down two equations that describe the ages of Luis and Christopher. Let Luis's current age be $l$ and Christopher's current age be $c$ $l = 3c$ $l = c + 12$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $c$ , and both of our equations have $l$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $3c$ $-$ $ (c + 12)$ which combines the information about $c$ from both of our original equations. Solving for $c$ , we get: $2 c = 12$ $c = 6$.